In a wireless communication system, particularly, in a mobile communication system, it has been studied to use a configuration called a centralized/cloud radio access network (C-RAN) in order to increase flexibility of base station deployment. In a C-RAN, functions of a base station are divided in two, and the functions are assigned to two devices called base band unit (BBU) and a remote radio head (RRH). The BBU and the RRH are connected by an optical fiber. However, in the C-RAN in which functions of a physical layer (PHY) including a baseband signal processing function are aggregated in the BBU and IQ data of radio signals are transmitted to the RRH, a very large bandwidth is necessary for an optical link between the BBU and the RRH called a mobile front haul (MFH). Therefore, it has been studied to reduce the bandwidth required for the MFH through redefining of functions to be assigned to the BBU and the RRH of the C-RAN.
FIG. 5 is a diagram showing a function division scheme called MAC-PHY Split. In the MAC-PHY Split scheme, functions of a media access control (MAC) layer or higher layers are assigned to a BBU, and functions of a physical layer are assigned to an RRH (Non-Patent Document 1). FIG. 6 is a diagram showing a functional division scheme called split-PHY processing (SPP). In the SPP scheme, functions of the MAC layer, higher layers, and an encoding function, which is one of physical layer functions, are assigned to the BBU, and the functions of the physical layer other than the encoding function are assigned to the RRH (Non-Patent Document 2).
In a mobile communication system, a scheduler which is one of the functions of the MAC layer generates control information for the functions of the physical layer. FIGS. 7 and 8 show a downlink signal transmission and a flow in the MAC-PHY Split scheme. FIG. 7 is a diagram showing the downlink signal transmission in the MAC-PHY Split scheme. A BBU 91 includes a scheduler 911 and a transmission unit 912. An RRH 92 includes a reception unit 921, an encoding unit 922 and a signal processing unit 923. FIG. 8 is a diagram showing a flow of the downlink signal transmission in the MAC-PHY Split scheme. In the downlink signal transmission, in the BBU 91, the transmission unit 912 transmits data to be transmitted to the terminal and the control information generated by the scheduler 911 to the RRH 92 via the MFH. In the RRH 92, the reception unit 921 receives the data and the control information. The encoding unit 922 and the signal processing unit 923 included in the functions of the physical layer read the control information received from the reception unit 921 and perform signal processing on the data according to an information included in the control information.
FIGS. 9 and 10 shows a downlink signal transmission and a flow in an SPP scheme. FIG. 9 is a diagram showing the downlink signal transmission in the SPP scheme. The BBU 93 includes a scheduler 931, an encoding unit 932 and a transmission unit 933. The RRH 94 includes a reception unit 941 and a signal processing unit 942. FIG. 10 is a diagram showing a flow of downlink signal transmission in the SPP scheme. In the downlink signal transmission, the BBU 93 outputs control information generated by the scheduler 931 to the encoding unit 932, and the transmission unit 933 transmits the control information output from the scheduler 931 to the RRH 94 via the MFH. The encoding unit 932 reads the control information output from the scheduler 931 and performs encoding on the data according to the information included in the control information. The encoding unit 932 outputs encoded data obtained by the encoding to the transmission unit 933. The transmission unit 933 transmits the encoded data output from the encoding unit 932 to the RRH 94 via the MFH. In the RRH 94, the reception unit 941 receives the control information and the encoded data, and the signal processing unit 942 performs signal processing on the encoded data according to the information included in the control information.
As shown in FIGS. 8 and 10, since a process in the RRH is performed on the basis of the control information received from the BBU, the process does not start unless the encoding unit or the signal processing unit completes the reading of the control information, and a waiting time occurs. In the downlink signal transmission in the function division scheme described above, there is a problem in that even when the data and the control information has been transmitted to the RRH via the MFH, a process in the physical layer cannot be started unless the reading of the control information is completed in the RRH, and a delay occurs by the waiting time caused by reading of the control information.